This invention relates to a battery operated power supply for an electroluminescent (EL) lamp and, in particular, to a driver having an H-bridge output in which the high side switches are not controlled by current mirrors.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a driver that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of drivers including an inductive boost circuit having an inductor in series with a switching transistor. Current through the inductor causes energy to be stored in a magnetic field around the inductor. When the current is abruptly shut off, the induced magnetic field collapses, producing a pulse of high voltage. The voltage across the inductor is proportional to L.multidot..sup.67 i /.sub.67 t. Thus, a low voltage at high current is converted into a high voltage at low current. The voltage on the lamp is pumped up by a series of high voltage pulses from the boost circuit.
The direct current produced by the driver must be converted into an alternating current in order to power an EL lamp. It is known in the art to use a switching bridge, called an H-bridge, to alternate the current through the lamp. The bridge operates like a double pole, double throw switch to alternate the polarity of the current through the lamp at a low frequency (200-1000 hertz).
It is known that an EL lamp can act like small electrostatic speaker when electrical pulses are applied to the lamp. It is also known to control the charge and discharge current through an EL lamp to minimize noise generated by the lamp. When a driver is implemented as an integrated circuit, the prior art uses current mirrors to control current through the switching transistors in an H-bridge, thereby reducing the noise generated by an EL lamp.
Although a substantial portion, or all, of an EL driver is typically implemented as an integrated circuit for reduced costs, further improvement in cost and efficiency is desired to meet market expectations of declining prices and improving performance. One measure of efficiency is the amount of power delivered to a lamp compared to the total power consumed by the driver. (Other measures of efficiency relate light output to power consumption and involve considerations not relevant here, such as lamp structure, pulse frequency, and wave shape.)
In view of the foregoing, it is therefore an object of the invention to improve the power efficiency of an EL driver.
Another object of the invention is to reduce the cost of an EL driver.
A further object of the invention is to reduce the current drawn by an EL driver.